Ieee 802.11overview

IEEE 802.11 Overview Mustafa Ergen [email_address] UC Berkeley

Standardization of Wireless Networks Wireless networks are standardized by IEEE. Under 802 LAN MAN standards committee. Application Presentation Session Transport Network Data Link Physical ISO OSI 7-layer model Logical Link Control Medium Access (MAC) Physical (PHY) IEEE 802 standards

IEEE 802.11 Overview Adopted in 1997. Defines; MAC sublayer MAC management protocols and services Physical (PHY) layers IR FHSS DSSS Goals To deliver services in wired networks To achieve high throughput To achieve highly reliable data delivery To achieve continuous network connection.

Components Station BSS - Basic Service Set IBSS : Infrastructure BSS : QBSS ESS - Extended Service Set A set of infrastrucute BSSs. Connection of APs Tracking of mobility DS – Distribution System AP communicates with another

Services Station services: authentication, de-authentication, privacy, delivery of data Distribution Services ( A thin layer between MAC and LLC sublayer) association disassociation reassociation distribution Integration A station maintain two variables: authentication state (=> 1) association state (<= 1)

Medium Access Control Functionality; Reliable data delivery Fairly control access Protection of data Deals; Noisy and unreliable medium Frame exchange protocol - ACK Overhead to IEEE 802.3 - Hidden Node Problem – RTS/CTS Participation of all stations Reaction to every frame

MAC Retry Counters Short retry counter Long retry counter Lifetime timer Basic Access Mechanism CSMA/CA Binary exponential back-off NAV – Network Allocation Vector Timing Intervals: SIFS, Slot Time, PIFS, DIFS, EIFS DCF Operation PCF Operation

PCF Operation Poll – eliminates contention PC – Point Coordinator Polling List Over DCF PIFS CFP – Contention Free Period Alternate with DCF Periodic Beacon – contains length of CFP CF-Poll – Contention Free Poll NAV prevents during CFP CF-End – resets NAV

Frame Types Protocol Version Frame Type and Sub Type To DS and From DS More Fragments Retry Power Management More Data WEP Order FC Duration /ID Address 1 Address 2 Address 3 Sequence Control Address 4 DATA FCS 2 2 6 6 6 2 6 0-2312 4 bytes NAV information Or Short Id for PS-Poll BSSID –BSS Identifier TA - Transmitter RA - Receiver SA - Source DA - Destination IEEE 48 bit address Individual/Group Universal/Local 46 bit address MSDU Sequence Number Fragment Number CCIT CRC-32 Polynomial Upper layer data 2048 byte max 256 upper layer header

Frame Subtypes RTS CTS ACK PS-Poll CF-End & CF-End ACK Data Data+CF-ACK Data+CF-Poll Data+CF-ACK+CF-Poll Null Function CF-ACK (nodata) CF-Poll (nodata) CF-ACK+CF+Poll Beacon Probe Request & Response Authentication Deauthentication Association Request & Response Reassociation Request & Response Disassociation Announcement Traffic Indication Message (ATIM) CONTROL DATA MANAGEMENT

Other MAC Operations Fragmentation Sequence control field In burst Medium is reserved NAV is updated by ACK Privacy WEP bit set when encrypted. Only the frame body. Medium is reserved NAV is updated by ACK Symmetric variable key WEP Details Two mechanism Default keys Key mapping WEP header and trailer KEYID in header ICV in trailer dot11UndecryptableCount Indicates an attack. dot11ICVErrorCount Attack to determine a key is in progress.

MAC Management Interference by users that have no concept of data communication. Ex: Microwave Interference by other WLANs Security of data Mobility Power Management

Authentication Authentication Prove identity to another station. Open system authentication Shared key authentication A sends B responds with a text A encrypt and send back B decrypts and returns an authentication management frame. May authenticate any number of station. Security Problem A rogue AP SSID of ESS Announce its presence with beaconing A active rogue reach higher layer data if unencrypted.

Association Association Transparent mobility After authentication Association request to an AP After established, forward data To BSS, if DA is in the BSS. To DS, if DA is outside the BSS. To AP, if DA is in another BSS. To “ portal ”, if DC is outside the ESS. Portal : transfer point : track mobility . (AP, bridge, or router) transfer 802.1h New AP after reassociation, communicates with the old AP.

Address Filtering More than one WLAN Three Addresses Receiver examine the DA, BSSID Privacy MAC Function WEP Mechanism

Power Management Independent BSS Distributed Data frame handshake Wake up every beacon. Awake a period of ATIM after each beacon. Send ACK if receive ATIM frame & awake until the end of next ATIM. Estimate the power saving station, and delay until the next ATIM. Multicast frame : No ACK : optional Overhead Sender Announcement frame Buffer Power consumption in ATIM Receiver Awake for every Beacon and ATIM

Power Management Infrastructure BSS Centralized in the AP. Greater power saving Mobile Station sleeps for a number of beacon periods. Awake for multicast indicated in DTIM in Beacon. AP buffer, indicate in TIM Mobile requests by PS-Poll

Synchronization Timer Synchronization in an Infrastructure BSS Beacon contains TSF Station updates its with the TSF in beacon. Timer Synchronization in an IBSS Distributed. Starter of the BSS send TSF zero and increments. Each Station sends a Beacon Station updates if the TSF is bigger. Small number of stations: the fastest timer value Large number of stations: slower timer value due to collision. Synchronization with Frequency Hopping PHY Layers Changes in a frequency hopping PHY layer occurs periodically (the dwell meriod). Change to new channel when the TSF timer value, modulo the dwell period, is zero

Scanning & Joining Scanning Passive Scanning : only listens for Beacon and get info of the BSS. Power is saved. Active Scanning: transmit and elicit response from APs. If IBSS, last station that transmitted beacon responds. Time is saved. Joining a BSS Syncronization in TSF and frequency : Adopt PHY parameters : The BSSID : WEP : Beacon Period : DTIM

Comb ining Management Tools Combine Power Saving Periods with Scanning Instead of entering power saving mode, perform active scanning. Gather information about its environments. Preauthentication Scans and initiate an authentication Reduces the time

The Physical Layer PLCP: frame exchange between the MAC and PHY PMD: uses signal carrier and spread spectrum modulation to transmit data frames over the media. Direct Sequence Spread Spectrum (DSSS) PHY 2.4 GHz : RF : 1 – 2 Mbps The Frequency Hopping Spread Spectrum (FHSS) PHY 110KHz deviation : RF : PMD controls channel hopping : 2 Mbps Infrared (IR) PHY Indoor : IR : 1 and 2 Mbps The OFDM PHY – IEEE 802.11a 5.0 GHz : 6-54 Mbps : High Rate DSSS PHY – IEEE 802.11b 2.4 GHz : 5.5 Mbps – 11 Mbps :

IEEE 802.11E EDCF - Enhanced DCF HCF - Hybrid Coordination Function QBSS HC – Hybrid Controller TC – Traffic Categories TXOP – Transmission Opportunity – granted by EDCF-TXOP or HC- poll TXOP AIFS – Arbitration Interframe Space

IEEE 802.11 Protocols IEEE 802.11a PHY Standard : 8 channels : 54 Mbps : Products are available. IEEE 802.11b PHY Standard : 3 channels : 11 Mbps : Products are available. IEEE 802.11d MAC Standard : operate in variable power levels : ongoing IEEE 802.11e MAC Standard : QoS support : Second half of 2002. IEEE 802.11f Inter-Access Point Protocol : 2 nd half 2002 IEEE 802.11g PHY Standard: 3 channels : OFDM and PBCC : 2 nd half 2002 IEEE 802.11h Supplementary MAC Standard: TPC and DFS : 2 nd half 2002 IEEE 802.11i Supplementary MAC Standard: Alternative WEP : 2 nd half 2002

WLAN Pending Issues Why 802.11a? Greater bandwidth (54Mb) Less potential interference (5GHz) More non-overlapping channels Why 802.11b? Widely available Greater range, lower power needs Why 802.11g? Faster than 802.11b (24Mb vs 11Mb)

Deployment Issues Re-purpose Symbol AP’s for secure admin services Deploy 802.11b with 802.11a in mind (25db SNR for all service areas) Delay migration to 802.11a until dual function (11b & 11a) cards become available

Frequency Bands- ISM Industrial, Scientific, and Medical (ISM) bands Unlicensed, 22 MHz channel bandwidth Extremely Low Very Low Low Medium High Very High Ultra High Super High Infrared Visible Light Ultra- violet X-Rays Audio AM Broadcast Short Wave Radio FM Broadcast Television Infrared wireless LAN 902 - 928 MHz 26 MHz Cellular (840MHz) NPCS (1.9GHz) 2.4 - 2.4835 GHz 83.5 MHz (IEEE 802.11) 5 GHz (IEEE 802.11) HyperLAN HyperLAN2

IEEE 802.11i Enhanced Security Agere/Microsoft/Agere/Cisco/Atheros/Intel/3Com/Intersil/Symbol/Certicom/RSA/Funk Key players Enhanced encryption software will replace WEP software; This is on a recommended best practice /voluntary basis; development in TgI: first draft Mar 2001; next draft due Mar 2002; stable draft: July 2002; final standard: Jan 2003 Status + Roadmap Mode of AES to use for encryption (CTR/CBC [CBC MIC] or OCB [MIC and Encryption function]) Key issues Actively proposing WEP improvement methods , participating in all official/interim meetings Agere’s activity Client and AP cards (Controller chip, Firmware, Driver) AP kernel, RG kernel, BG kernel Products affected This applies to 802.11b, 802.11a and 802.11g systems. 802.1x is key reference for upper layer authentication Related standards High: weakness of WEP encryption is damaging the 802.11 standard perception in the market Importance Enhancements to the 802.11 MAC standard to increase the security; addresses new encryption methods and upper layer authentication Description

IEEE 802.1 X - Port Based Control Microsoft/Cisco/Certicom/RSA/Funk Key players Standard available – Spring 2001 Status + Roadmap Home in IETF for EAP method discussions Key issues Adding EAP auth types to products Agere’s activity Supported in AP-2000, AP-1000/500, Clients (MS drivers for XP/2000 beta) Products affected This applies to 802.11b, 802.11a and 802.11g systems Related standards High: forms a key part of the important 802.11i proposals for enhanced security Importance A framework for regulating access control of client stations to a network via the use of extensible authentication methods Description

IEEE 802.1 p - Traffic Class N/A Key players Final standard; incorporated in 1998 edition of 802.1d (annex H) Status + Roadmap N/A Key issues Investigating implementation options Agere’s activity Client and AP cards (Driver ) ; AP kernel, RG kernel, BG kernel Products affected This applies to 802.11b, 802.11a and 802.11g systems; i s an addition to the 802.1d Bridge standard (annex H). Related standards Medium : forms a key part of the 802.11e proposals for QoS at the MAC level Importance A method to differentiate traffic streams in priotity classes in support of quality of service offering Description IEEE 802.1 p ( Traffic Class and Dynamic Multicast Filtering ) Reference

Glossary of 802.11 Wireless Terms, cont. BSSID & ESSID: Data fields identifying a stations BSS & ESS. Clear Channel Assessment (CCA): A station function used to determine when it is OK to transmit. Association: A function that maps a station to an Access Point. MAC Service Data Unit (MSDU): Data Frame passed between user & MAC. MAC Protocol Data Unit (MPDU): Data Frame passed between MAC & PHY. PLCP Packet (PLCP_PDU): Data Packet passed from PHY to PHY over the Wireless Medium.

Overview, 802.11 Architecture STA STA STA STA STA STA STA STA AP AP ESS BSS BSS BSS BSS Existing Wired LAN Infrastructure Network Ad Hoc Network Ad Hoc Network

Frequency Hopping and Direct Sequence Spread Spectrum Techniques Spread Spectrum used to avoid interference from licensed and other non-licensed users, and from noise, e.g., microwave ovens Frequency Hopping (FHSS) Using one of 78 hop sequences, hop to a new 1MHz channel (out of the total of 79 channels) at least every 400milliseconds Requires hop acquisition and synchronization Hops away from interference Direct Sequence (DSSS) Using one of 11 overlapping channels, multiply the data by an 11-bit number to spread the 1M-symbol/sec data over 11MHz Requires RF linearity over 11MHz Spreading yields processing gain at receiver Less immune to interference

802.11 Physical Layer Preamble Sync, 16-bit Start Frame Delimiter, PLCP Header including 16-bit Header CRC, MPDU, 32-bit CRC FHSS 2 & 4GFSK Data Whitening for Bias Suppression 32/33 bit stuffing and block inversion 7-bit LFSR scrambler 80-bit Preamble Sync pattern 32-bit Header DSSS DBPSK & DQPSK Data Scrambling using 8-bit LFSR 128-bit Preamble Sync pattern 48-bit Header

802.11 Physical Layer, cont. Antenna Diversity Multipath fading a signal can inhibit reception Multiple antennas can significantly minimize Spacial Separation of Orthoganality Choose Antenna during Preamble Sync pattern Presence of Preamble Sync pattern Presence of energy RSSI - Received Signal Strength Indication Combination of both Clear Channel Assessment Require reliable indication that channel is in use to defer transmission Use same mechanisms as for Antenna Diversity Use NAV information

Performance, Theoretical Maximum Throughput Throughput numbers in Mbits/sec: Assumes 100ms beacon interval, RTS, CTS used, no collision Slide courtesy of Matt Fischer, AMD

Ieee 802.11overview

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